<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Event</title>
</head>
<body>
    <button id="btn1">按钮1</button>
    <button id="btn2">按钮2</button>
    <button id="btn3" onclick="demo()">按钮3</button>

    <script type="text/javascript">
        let btn1 = document.getElementById('btn1')
        btn1.addEventListener('click', () => {
            alert('按钮1被点击了')
        })

        let btn2 = document.getElementById('btn2')
        btn2.onclick = () => {
            alert('按钮2被点击了')
        }

        function demo() {
            alert('按钮3被点击了')
        }
    </script>
</body>
</html>